This invention relates to the combined functions of pumping liquids and liquid slurries and the preliminary comminution and grinding of solids and semi-solids contained in the product prior to its being pumped so as to minimize the danger of clogging in the pump and pressure lines. More particularly, the invention relates to an improved rotary pumping and grinding unit wherein a centrifugal type pump impeller and a rotary grinder and comminutor mechanism are driven by a common rotary shaft and arranged in cooperating successive relationship.
The invention has particular utility in connection with pressure sewage systems wherein a slurry of liquid sewage with some solid and semi-solid material therein, stored in a sewage collection tank, is to be pumped through pressure sewage lines and consequently must be ground and comminuted prior to delivery to a centrifugal pump stage in order to avoid clogging of the pump and pressure lines.
The type of submersible rotary grinding and pumping units to which the invention relates generally have a sealed motor housing and an electrical motor with a vertical, downwardly extending rotary output shaft that extends through a surrounding annular stator section for the grinder and centrifugal impeller sections. The material enters the unit through a downwardly facing axial inlet spaced, for example, about two inches from the bottom of the tank and a cutter or shredder bar or other rotary grinding member is mounted at the lower end of the rotary shaft in the path of the material to be pumped. The centrifugal impeller for the pumping section is mounted above the grinder member and is adapted to create a suction to draw the product through the grinder section and propel it radially outward to a circumferential volute chamber that terminates in an outlet pipe.
In the case of pressure sewage systems, the grinder or shredder section must comminute such solid material as bone, sticks, glass, bottle caps, cans, nylon garments, rags, wood and similar foreign matter that generally loads up and clogs conventional pumps. The problem of clogging can occur, for example, where the liquid product makes a 90.degree. turn from axial to radial flow at the point where the liquid passes from the grinder section to the centrifugal pump section. The abrupt directional change creates turbulence and reduces the output pressure head.
Another disadvantage inherent in prior art, pumps with shredders and abraders of the type described is that these devices are vulnerable to abrasive action and chipping of cooperating pump surfaces due to the highly abrasive material that may be contained in the product to be pumped. The same abrasive action is apt to cause wear of the impeller and stator surfaces and eventually reduce the output pressure due to improper clearances between the two cooperating surfaces.
The apparatus of the present invention reduces the difficulties described above and affords other features and advantages heretofore not obtainable.